MCS biodiversity programme manager Peter Richardson said: “This year, we received our first reports of the huge but harmless barrel jellyfish off North Wales back in early January, and this species has occurred in huge numbers in the Irish Sea and beyond ever since, with reports received from North Somerset to the Firth of Clyde.

"Since May we have also received reports of large numbers of several other species of jellyfish from various coastal all sites round the UK.”

Dr Richardson said Wales was one of the most rewarding parts of the UK for jellyfish study, with more reports and more species.

Tremadog Bay, Carmarthen Bay and Anglesey were key locations for jellyfish, with increasing numbers of larger groups being spotted in recent years.

“Last year was phenomenal, the biggest in the history of our survey,” said Dr Richardson.

“But we’ve already had 500 sightings and we’re only halfway through the summer season.”